The transportation of hydrocarbons produced from subsea wells is an integral part of offshore hydrocarbon production operations. In a typical offshore production arrangement, as shown in FIG. 1, a plurality of subsea wells (not shown) are drilled from an offshore drilling and production platform 1. Once the targeted formations (not shown) have been reached through drilling operations, production tubing (not shown) is typically set in place and further procedures including, for example, the perforation of the production tubing in selected target zones, are performed to produce hydrocarbons from the well. The offshore drilling and production platform 1 may include storage facilities 2 for the temporary storage of hydrocarbons produced from wells (not shown).
Delivery of hydrocarbons produced from subsea wells may be performed with any of several techniques known in the art. For example, hydrocarbons may be produced from a remote subsea wellhead (e.g., the subsea wellhead may be positioned at a subsea location that is some distance away from the location of the offshore drilling and production platform 1) and then piped to the offshore drilling and production platform 1. Alternatively, the hydrocarbons produced from subsea wells may be routed from a subsea wellhead directly to land locations through a pipeline, as long as the wellhead is located sufficiently close to the shore. Hydrocarbons may also be produced from a subsea well and then transferred through a pipeline 3 to a moored floating production, storage, and offloading tanker (FPSO tanker) 4.
Regardless of the technique used to deliver hydrocarbons from the well to transportation facilities, the hydrocarbons must be processed before being transported to other facilities for further refining or delivery. An important requirement for the transportation of liquid hydrocarbons is that low pressure flash gas must be removed from the liquid hydrocarbons so that the vapor pressure of the liquid hydrocarbons is reduced. Reduced vapor pressure ensures that low pressure flash gas will not xe2x80x9cevolve,xe2x80x9d or come out of solution, while the hydrocarbons are being transported. For standard tanker transport, the vapor pressure of the transported hydrocarbons must be near or below atmospheric pressure (14.7 psia) in order to minimize gas evolution during loading of the tanker. Evolving flash gas can pose a safety hazard for transporters and for the environment.
Hydrocarbon processing to remove low pressure flash gas may be performed at land based facilities for producing wells that are located close to the shore. Deepwater wells that are located significant distances offshore provide a more complicated processing issue. For example, processing of hydrocarbons to remove low pressure flash gas may be performed at production platforms or at FPSOs. Processing at either of these locations, however, requires the presence of shuttle tankers to transport the processed hydrocarbon (e.g., the hydrocarbon with a reduced vapor pressure) to other facilities. For example, production platforms and FPSOs have a limited storage capacity for holding processed hydrocarbons. The problem is particularly relevant for FPSOs because the storage capacity of an FPSO is limited by the volume of the hold of the ship (which is further limited because of the space required to house the processing equipment). Thus, if shuttle tankers are unavailable due to, for example, poor weather conditions, processing must be interrupted when all storage facilities are full. Processing interruptions or slowdowns may have adverse economic consequences because of the high cost of operating and maintaining offshore facilities.
An alternative method includes processing hydrocarbons with a subsea separation system. Subsea separation systems are known in the art, and staged separation is the most common technique used in the industry for hydrocarbon stabilization. Preferably, pressure at the last stage of a staged separation system is at or near atmospheric pressure so that a desired level of hydrocarbon stabilization may be achieved without excessive heating (e.g., because heating liquid hydrocarbons increases the vapor pressure and facilitates removal of flash gas). Therefore, flash gas extracted from the hydrocarbons in the last stage of the staged separation system is typically also at or near atmospheric pressure. Generation of the low pressure at the last stage of separation typically requires the use of a mechanical compressor installed near the separation equipment on the seafloor because the separation system is located at great depth. The mechanical compressor typically requires an independent power source, and the mechanical compressor must be regularly maintained. Further, the low pressure flash gas may require a pressure boost to provide a pressure differential so that the flash gas can overcome pipe friction and the static pressure in a pipeline and flow to the surface (e.g., the ocean surface). As a result, prior art systems typically include boosting the pressure of the extracted low pressure flash gas with a mechanical compressor. Once the low pressure flash gas reaches the surface, the flash gas may be disposed of via a flare or may be separately transported.
An alternative to boosting the flash gas pressure with a compressor is to forego subsea processing and transport the hydrocarbon with an elevated vapor pressure directly to the surface. Transport to the surface is facilitated by operating the last stage of the subsea separation process at a sufficiently high pressure to provide the necessary pressure differential required to boost the hydrocarbon to the surface. However, this xe2x80x9clivexe2x80x9d hydrocarbon still contains flash gas at a higher than atmospheric pressure and the xe2x80x9clivexe2x80x9d hydrocarbon must be processed, as previously mentioned, before being loaded onto shuttle tankers.
One aspect of the invention is a system for subsea flash gas compression comprising a first separator adapted to remove high pressure flash gas from a hydrocarbon product and a second separator is adapted to remove low pressure flash gas from the hydrocarbon product after removal of the high pressure flash gas therefrom. An ejector is coupled at a high pressure input thereof to a high pressure flash gas output of the first separator and at a low pressure input thereof to a low pressure flash gas output of the second separator. An output of the ejector is coupled to an outlet pipeline extending from proximate the sea bottom to the sea surface.
In another aspect, the invention comprises a method of separating flash gas from a hydrocarbon product. The method comprises separating high pressure flash gas from the hydrocarbon product in a first separator, and a flash gas output of the first separator is coupled to a high pressure input of an ejector. Low pressure flash gas is separated from the hydrocarbon product after removal of the high pressure flash gas therefrom in a second separator, and a flash gas output of the second separator is coupled to a low pressure input of the ejector. An output of the ejector is conducted from proximate the second separator on the sea bottom to the sea surface.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.